I've reinstalled Vivaldi and i'm up and running again. I'm trying to find out the cause of this 'freezing'.

Short Update:
The last website I've visited before the freeze, is an intranet website using a username and password to log into the site.
After visiting this website I quited Vivaldi. After starting it up again, Vivaldi 'freezes''.

I noticed this as well. When switching back to Vivaldi from another app (via Cmd + Tab, or simply clicking the icon in the doc), Vivaldi doesn't always reappear. I think it may only occur after explicitly hiding the app with Cmd + H, but can not confirm. The only solution has been to hide all of my other windows so that Vivaldi is visible again, then I can select it.